1. Field of the invention.
The present invention relates to a process for production of a colour image by the successive transfer in registration onto a common carrier of distinct colour images each formed on a corresponding support and to a colour-image-forming apparatus of the type having a transfer device for transferring in registration distinct colour images which have been formed on individual first supports to a second support, i.e. a common carrier.
The invention concerns in particular colour proofing and colour printing.
2. Description of the prior art.
The process of transforming a multicolour original into an accurate and pleasing colour reproduction involves a series of complex manipulations: conversion of continuous tones into halftones; separation of multicolour images to individual films for the three primary colours and black; and adjustment of colour balance, size and composition. Prepress colour proofing systems make it possible to check the accuracy of these transformations without the high cost and time delay of on-press proofing.
One known colour proofing system is based on the production of distinct colour images by exposing precoloured films to corresponding colour separation films and then laying down all the colour images in register on one single support. Such a system is suited for the production of a few colour reproductions only, as all the different steps require manual intervention from a human operator.
The other extreme in colour reproduction is formed by printing presses in which a paper web is passed at high speed past different colour printing cylinders which are part of a rigid and accurate machine construction whereby the required precision in the image registering is obtained. In other types of printing machines, reference marks on the paper co-operate with sensing and steering mechanisms to obtain the correct registration of the (four) part images on the paper support which is afterwards cut into sheets.